Differences in endo-β-1,4-xylanase functionality result from the different ability of the enzymes to convert water-unextractable arabinoxylan (WU-AX) into solubilised AX (S-AX) and to degrade water-extractable arabinoxylan (WE-AX) and S-AX to lower molecular weight. These respective endoxylanase activities and their relative proportions were determined for two endoxylanases from Bacillus subtilis and Aspergillus aculeatus, according to a generally applicable procedure. Isolated wheat WE-AX and WU-AX containing wheat squeegee starch were prepared and incubated with different levels of endoxylanases. The supernatants were analysed for reducing xylose (RX) formed and WU-AX solubilised. Molecular weights of the products formed were assessed. Relative activities differed widely for the two endoxylanases. Based on the results and additional enzyme-substrate binding experiments, mechanisms possibly accounting for these differences are discussed.